The melanocortins represent a group of peptides which derive from the same precursor, proopiomelanocortin (POMC), and which are structurally close: the adrenocorticotropic hormone (ACTH), the α-melanocyte stimulating hormone (α-MSH), β-MSH and γ-MSH (Eipper B. A. and Mains R. E., Endocr. Rev. 1980, 1, 1-27). The melanocortins carry out numerous physiological functions. They stimulate the synthesis of the steroids by the adrenal cortex and the synthesis of eumelanine by the melanocytes. They regulate the intake of food, energetic metabolism, sexual function, neuronal regeneration, blood pressure and heart rate, as well as the perception of pain, learning, attention and memory. The melanocortins also possess anti-inflammatory and anti-pyretic properties and control the secretion of several endocrine or exocrine glands such as the sebaceous, lacrimal and mammary glands, the prostate and the pancreas (Wikberg J. E. et al. Pharmacol. Res. 2000, 42, 393-420; Abdel-Malek Z. A., Cell. Mol. Life. Sci. 2001, 58, 434-441).
The effects of the melanocortins are mediated by a family of membrane receptors specific to seven transmembrane domains and coupled to G proteins. Five sub-types of receptors, named MC1 to MC5, have been cloned and characterized to date. These receptors differ in their tissue distribution and in the affinity of different melanocortins, the MC2 receptors only recognizing the ACTH. The stimulation of the melanocortin receptors activates adenylate cyclase with production of cyclic AMP. If the specific functional roles of each of the receptors are not totally elucidated, the treatment of pathological disorders or diseases can be associated with an affinity for certain sub-types of receptors. Thus activation of the MC1 receptors has been associated with the treatment of inflammations, while blocking them has been associated with the treatment of cutaneous cancers. The treatment of nutritional disorders has been associated with the MC3 and MC4 receptors, the treatment of obesity with agonists and the treatment of cachexia and anorexia with antagonists. Other indications linked to the activation of the MC3 and MC4 receptors are problems with sexual activity, neuropathic pain, anxiety, depression and drug addiction. The activation of the MC5 receptors has been associated with the treatment of acne and dermatoses.